A variety of industrial applications exist where power ultrasonic actuators, such as ultrasonic horns, are used to produce large amplitude vibrations. These applications include medical/surgical, automotive, food preparation, and textile cutting applications, as well as use in fabrication industries and material joining Ultrasonic actuators are attractive for their ability to generate precision high strokes, torques and forces while operating under relatively harsh conditions, such as temperatures in the range of single digit Kelvin to 1273 Kelvin. Details related to a variety of applications can be found in the following references: A. Shoh, “Industrial Applications of Ultrasound—A review 1. High Power Ultrasound”, IEEE Trans on Sonics and Ultrasonics, SU-22, 2, pp. 60-71, 1975; L. Parrini, “New Methodology For The Design Of Advanced Ultrasonic Transducers For Welding Devices”, Proceedings of the IEEE International Ultrasonics Symposium, pp. 699-714, 2000; W. W. Cimino, L. J. Bond, “Physics of Ultrasonic Surgery Using Tissue Fragmentation, Proceedings of the IEEE International Ultrasonics Symposium, pp. 1597-1600, 1995; K. F. Graf, Process Applications of Power Ultrasonics—A Review”, Proceedings of the IEEE International Ultrasonics Symposium, pp. 628-641, 1974.
Known in the prior art is Stegelmann, U.S. Pat. No. 7,754,141, issued Jul. 13, 2010, which is said to disclose in one aspect an ultrasonic horn for transporting ultrasonic energy to an operating location defining a radial direction and an axial direction. The ultrasonic horn includes a horn member and an energy transfer surface disposed on the horn member. The ultrasonic horn also includes an axle member joined to the horn member where the axle member is provided by a first material. The ultrasonic horn further includes an isolation member integrally joined to the axle member and adapted for mounting the ultrasonic horn at a work location where at least a portion of the isolation member is provided by a second material.
Stegelmann further discloses that the first material and the second material can exhibit different properties for transporting ultrasonic energy. In a particular aspect, the axial isolation submember can be acoustically decoupled from the axle member. According to Stegelmann, configuring the isolation member in this manner provides several advantages. For instance, the isolation member will suitably transfer a reduced amount of vibration to the coupler. Accordingly, this can advantageously decrease noise associated with the ultrasonic horn, improve performance due to the lower vibration and improve the mounting of the horn. Moreover, equipment life of both the horn and the coupler can be improved.
As is known in the prior art, the application of pre-stress or preloading can be used to maintain the integrity of the piezoelectric material in an actuator. Known in the prior art is Chambers et al., U.S. Pat. No. 7,327,637, issued Feb. 5, 2008 (herein after referred to as the Chambers et al. '637 patent) and an article entitled “Characterization of piezoelectrically induced actuation of Ni—Mn—Ga singe crystals” by Chambers et al. (Smart Structures and Materials 2005: Active Materials: Behavior and Mechanics, edited by William D. Armstrong, Proceedings of SPIE Vol. 5761 (SPIE, Bellingham, Wash., 2005), pages 478-489) (herein after referred to as the Chambers et al. Induced Actuation article). These references are said to disclose in part the actuation of magnetic materials using stress waves. In particular the Chambers et al. '637 patent discloses an acoustic actuator, including an acoustic stress wave generator and an actuation material operatively positioned relative to the acoustic stress wave generator for the delivery of acoustic stress waves from the generator to actuation material. The application of a pre-stress is shown and discussed with respect to FIG. 6 of the Chambers et al. '637 patent and FIG. 1 of the Chambers et al. Induced Actuation article (herein reproduced as FIG. 1). Also disclosed is that the clamp surrounding the piezoelectric stack (the moonie clamp) holds and positions the stack, without the use of a bonding agent. It also provides a prestress on the stack, which improves its performance. The Moonie clamp can be driven in the transverse direction. In this type of actuator, when the stack expands the work surface contracts and when the stack contracts the work surface expands. In the structures disclosed in the Chambers et al. '637 patent and Induced Actuation article, the clamping function but not the transverse amplification is employed. Additional details regarding the Moonie clamp can be found in the articles entitled “Metal-Ceramic Composite Transducer, the ‘Moonie’” by Onitsuka et al. (Journal of Intelligent Material Systems and Structures, July 1995, Vol. 6 No. 4, pages 447-455), “Flextensional ‘Moonie’ Actuator” by R. E. Newnham et al. (1993 Ultrasonics Symposium, pages 509-513, IEEE, 1051-0117/93/0000-0509), and “Hallow Piezoelectric Composites” by J. F. Fernandez et al. (Sensors and Actuators A, 51, pages 183-192 (1996)).
The structures in the Chambers et al. '637 patent and Induced Actuation article are directed to low power and low frequency applications, principally below 100 Hz, and are well suited for micropositioning applications, as the patent states. If stress levels in addition to those provided by the piezoelectric stack are required, the Chambers et al. '637 patent discloses that a separate acoustic horn can be placed between the piezoelectric material and the actuation material.
In generating stress waves at low frequencies, the devices disclosed in the Chambers et al. '637 patent and Induced Actuation article are not driven at resonance. In fact resonances are described by the Chambers et al. '637 patent and Induced Actuation article as poorly understood unwanted artifacts. When discussing the measured propagated stress wave in the FSMA actuation material resulting from the control voltage pulse plotted in FIG. 7A of the Chambers et al. '637 patent, the patent discloses that it is found that this stress wave is not ideal, reaching nearly the same tensile stress as compressive stress. The oscillations along the peak of the stress wave are disclosed to be due to the length of the input control pulse relative to the period of the resonance of the piezoelectric stack. The Chambers et al. '637 patent indicates that a sound wave can travel from one end of the stack to the other approximately 7 times in 50 μs, based on the speed of sound calculated from stack properties, resulting in the 7 small peaks seen along the major peak of the wave. In discussing the effect of repetition rate on peak-to-peak output stress on a full actuation cycle, the Chambers et al. Induced Actuation article states that the data shows a peak at 70 Hz, along with several other local minima and maxima. They disclose that they believe these features are associated with acoustic resonances in the actuator itself, or the spring load system. However, they state that the details of the resonances are not understood well.
Pre-stressing the material becomes especially beneficial when the piezoelectric material is driven at high power. Barillot et al, U.S. Pat. No. 6,927,528, issued Aug. 9, 2005, is said to disclose in part the damping capacities of a piezoelectric actuator and its resistance to dynamic external stresses. One embodiment shown in Figure. 7 in Barillot, herein reproduced as FIG. 2, discloses that to increase the capacity of the actuator to resist higher external stresses, the preloading of the piezoelectric components can also be increased. It is further disclosed that this preloading is normally performed by the actuator shell 31, but its value is limited in practice by the elastic limit of the material of the shell 31. Barillot further states that it may therefore be advantageous to add an additional preloading device 35 arranged in parallel to the large axis 16 of the actuator so as to increase the capacity of the actuator to resist external stresses. To use such a system, the use of an extruded shell 31 is indicated as being particularly useful. Barillot states that two springs can be connected to the shell 31 along its large axis to increase the preloading on the piezoactive components.
Another approach also known in the prior art to pre-stressing piezoelectric material in high power ultrasonic actuators is the use of a stress bolt as shown in FIG. 3A. Typically these actuators are assembled with a horn, piezoelectric rings, backing and a stress bolt. The ring elements are connected electrically in parallel and placed between the horn and the backing ring. The pre-stress bolt is inserted through a backing ring and the piezoelectric rings and screwed into the horn until a desired pre-stress level, such as greater than 20 MPa, is reached. Another example of pre-stressing using a bolt is disclosed in the international application identified by the World International Patent Organization International Publication Number WO 03/026810 A1, which example is reproduced as FIG. 3B. This document discloses in part that a piezoelectric element 1 comprising two piezoelectric ceramic rings and thin electrodes is held in compression by a pre-load between a steel rear mass 2 and the horn 3. The pre-load is provided by a pultruded glass fibre tube 4 under compression in a sliding fit with a high-tensile cap-head bolt 5. A washer 6 is inserted between the head of the bolt and the rear face of the pultruded tube. The cap head bolt is screwed into the transducer horn adjacent to the piezoelectric ceramic at 7, securing the arrangement. Tightening the cap-head bolt 5 forces the pultruded tube 4 to remain under compression. The washer 6 ensures that the torsional force on the tube and consequently on the piezoelectric ceramic is kept to a minimum. Also known in the prior are is the use of an insulating bolt to address the issue of internal electrical discharge or to eliminate the need for an insulating covering over a traditional metallic bolt. For example the insulating bolt can be made of E glass which has a compliance that is twice that of steel and nearly as strong.
A number of problems in ultrasonic horns that are constructed with a pre-stress bolt have been observed. One of these problems is that ultrasonic horns with pre-stress bolts are susceptible to electrical discharge and mechanical failure. In addition typically ultrasonic horns include numerous components. A high component count leads to the result that their design, manufacture, assembly and integration into other structures can be complicated and costly. It can also be expensive and time consuming to make an actuator element having a hole defined therein that is designed to allow the pre-stress bolt to pass through. The hole defined in an actuator element can also be a “stress raiser” which can lead to mechanical failure of the actuator.
There is a need for an ultrasonic horn that addresses the issues of fabrication and assembly complexity and as well as performance failure issues.